Cellular respiration
is the process of extracting energy in the form of
ATP
from the glucose in the food you eat. How does cellular respiration happen inside of the cell? Cellular respiration is a three step process. Briefly:

In stage one, glucose is broken down in the cytoplasm of the cell in a process called
glycolysis
.

In stage two, the pyruvate molecules are transported into the mitochondria. The
mitochondria
are the organelles known as the energy "powerhouses" of the cells (
Figure
below
). In the mitochondria, the pyruvate, which have been converted into a 2-carbon molecule, enter the
Krebs cycle.
Notice that mitochondria have an inner membrane with many folds, called
cristae
. These cristae greatly increase the membrane surface area where many of the cellular respiration reactions take place.

In stage three, the energy in the energy carriers enters an
electron transport chain
. During this step, this energy is used to produce ATP.

Oxygen is needed to help the process of turning glucose into ATP. The initial step releases just two molecules of ATP for each glucose. The later steps release much more ATP.

Most of the reactions of cellular respiration are carried out in the mitochondria.

The Reactants

What goes into the cell? Oxygen and glucose are both
reactants
of cellular respiration. Oxygen enters the body when an organism breathes. Glucose enters the body when an organism eats.

The Products

What does the cell produce? The
products
of cellular respiration are carbon dioxide and water. Carbon dioxide is transported from your mitochondria out of your cell, to your red bloodcells, and back to your lungs to be exhaled. ATP is generated in the process. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. This only occurs in the presence of oxygen.

The Chemical Reaction

The overall chemical reaction for cellular respiration is one molecule of glucose (C
6
H
12
O
6
) and six molecules of oxygen (O
2
) yields six molecules of carbon dioxide (CO
2
) and six molecules of water (H
2
O). Using chemical symbols the equation is represented as follows:

Stage one of cellular respiration is glycolysis. Glycolysis is the splitting, or
lysis
of glucose. Glycolysis converts the 6-carbon glucose into two 3-carbon
pyruvate
molecules. This process occurs in the cytoplasm of the cell, and it occurs in the presence or absence of oxygen. During glycolysis a small amount of NADH is made as are two ATP. The NADH temporarily holds energy, which will be used in stage three.

In the presence of oxygen, under
aerobic
conditions, pyruvate enters the mitochondria to proceed into the Krebs cycle. The second stage of cellular respiration is the transfer of the energy in pyruvate, which is the energy initially in glucose, into two energy carriers, NADH and FADH
2
. A small amount of ATP is also made during this process. This process occurs in a continuous cycle, named after its discover, Hans Krebs. The Krebs cycle uses a 2-carbon molecule (acetyl-CoA) derived from pyruvate and produces carbon dioxide.

Stage three of cellular respiration is the use of NADH and FADH
2
to generate ATP. This occurs in two parts. First, the NADH and FADH
2
enter an electron transport chain, where their energy is used to pump, by active transport, protons (H
+
) out of the thylakoid. This establishes a proton gradient across the thylakoid membrane. These protons then flow back into the thylakoid by facilitated diffusion. During this process, ATP is made by adding inorganic phosphate to ADP. For each glucose that starts cellular respiration, in the presence of oxygen (aerobic conditions), 36-38 ATP are generated. Without oxygen, under
anaerobic
conditions, much less (only two!) ATP are produced.

Vocabulary

aerobic
: In the presence of oxygen.

anaerobic
: In the absence of oxygen.

ATP
: Usable form of energy inside the cell; adenosine triphosphate.

cellular respiration
: Process of breaking down glucose to obtain energy in the form of ATP.

cristae
: Inner membrane folds of the mitochondrion.

electron transport chain
: Series of electron-transport molecules that pass high-energy electrons from molecule to molecule and capture their energy.